1. Field of the Invention
The present invention relates to piezoelectric filters, and more particularly relates to balanced input-output piezoelectric filters with improved voltage utilization efficiency and noise resistance.
2. Description of the Related Art
In general, conventional balanced input-output piezoelectric filters using a surface acoustic wave (SAW) provided in a video IF amplifying circuit in a television receiver or other such apparatuses are well known. However, filters utilizing a bulk wave of a piezoelectric substrate, such as a 4.5 MHz-piezoelectric filter provided in an audio IF amplifying circuit in the television receiver and a 10.7-MHz piezoelectric filter provided in an IF amplifying circuit in an FM receiver, are not designed for balanced input and output.
Specifically, conventional piezoelectric filters are unbalanced input-output piezoelectric filters to and from which an unbalanced signal is input and output. Thus, the conventional piezoelectric filters have low voltage utilization efficiency and are influenced by common mode noise having the same phase with respect to a ground.
To overcome the problems described above, preferred embodiments of the present invention provide a balanced input-output piezoelectric filter with greatly improved voltage utilization efficiency and noise resistance.
According to one preferred embodiment of the present invention, a balanced input-output piezoelectric filter includes a ground electrode provided at the approximate center of a piezoelectric substrate in the thickness direction and at least one of an input electrode and an output electrode provided on both major surfaces of the piezoelectric substrate so as to be opposed to the ground electrode, thus providing a balanced-circuit connecting electrode. The piezoelectric substrate is vibrated in a thickness expansion vibration mode.
At least one of the input electrode and the output electrode provided on both major surfaces of the piezoelectric substrate functions as a balanced-circuit connecting input electrode or a balanced-circuit connecting output electrode. When the piezoelectric substrate is polarized in two opposite directions with the ground electrode provided therebetween as a boundary, the piezoelectric substrate expands and contracts in the thickness direction such that one side from the ground electrode expands while the other side contracts. Hence, the piezoelectric substrate vibrates in a double-wave mode. When the piezoelectric substrate is polarized in the same direction with the ground electrode provided as the boundary, both sides of the piezoelectric substrate expand and contract together. Hence, the piezoelectric substrate vibrates in a fundamental-wave mode.
According to a preferred embodiment of the present invention, the ground electrode is provided in the piezoelectric substrate, and at least one of the input electrode and the output electrode is provided on both principal planes of the piezoelectric substrate. Therefore, the piezoelectric filter for balanced input and output is obtained. As a result, a balanced input-output piezoelectric filter having improved voltage utilization efficiency and noise resistance against common mode noise is obtained even when the filter uses a bulk wave of the piezoelectric substrate.
Other features, characteristics, elements and advantages of the present invention will become apparent from the following description of preferred embodiments thereof with reference to the attached drawings.